Organic carbodiimides and their use as additives to aqueous polymer dispersions are known. They are added, for example, to polymer dispersions in order to increase the molecular weight of the polymers. In order to be able to disperse the carbodiimides simply and homogeneously in the dispersion, they are provided with hydrophilic groups.
EP-A-198 343 describes carbodiimides which carry sulfonate groups and also, if desired, polyethylene oxide units.
EP-A-686 626, moreover, discloses carbodiimides in which the hydrophilicity is brought about by ammonium groups, which are introduced by way of dialkylamino alcohols, by sulfonate groups, which are introduced by way of salts of hydroxy-functional alkylsulfonic acids, or by polyethylene oxide radicals.
The abovementioned products, however, have the following disadvantages:
Cationic products, such as carbodiimides hydrophilicized by ammonium groups, are incompatible with the anionically stabilized dispersions that are usually used.
The carbodiimides hydrophilicized with sulfonate groups are difficult to prepare. Owing to the highly lipophobic nature of the starting salts used, the reaction with the hydrophobic isocyanato-containing precursors is extremely difficult, since their mutual solubility is very low.
The dispersions cured using carbodiimides hydrophilicized with polyalkylene oxide radicals possess an undesirable permanent hydrophilicity.
DE-A-19821668 discloses carbodiimides based on 1,3-bis(1-methyl-1-isocyanatoethyl)benzene in which the hydrophilicization is brought about using amino sulfonic acids.
DE-A-19954006, unpublished at the priority date of the present specification, discloses carbodiimides based on aliphatic or aromatic polyisocyanates where the hydrophilicization is brought about using amino carboxylic acids.
During recent years, increasing environmental concerns over the elimination of VOC's (Volatile Organic Content) in consumer products have accelerated the growth in the importance of latex dispersions for the adhesives and coatings markets. The pursuit to replace solvent-based coatings with more environmentally friendly water based ones, however, has rendered the latter with physical and mechanical properties, such as tensile strength, chemical and abrasion resistance, and toughness, that are not as good as solvent-based coatings. In addition, the appearance of applied latex coatings is observed to be significantly duller than solvent borne ones. These inferior properties are due to the reduced film formation of the latex, which is exacerbated by premature crosslinking. In an attempt to remedy these limitations, there has been a continued interest in discovering and introducing novel crosslinking methods for functionalized latex polymers, which do not compete with film formation yet result in tough films.
Many practical applications of polymer dispersions require the post-cross-linking of polymer films after application to the substrate in order to enhance the mechanical properties of the film such as film-cohesion, solvent resistance, scrub resistance etc. On the one hand, the crosslinking reaction should be efficient and fast at ambient conditions. On the other hand, the reaction should be suppressed during storage to provide for a sufficiently long pot-life of the dispersion. Up to now, a variety of cross-linking systems have been described in the literature that are more or less compliant with these contradictory requirements. Functional groups currently used are aziridine, epoxy, isocyanate, keto, acetoacetoxy, and carbodiimide groups. Of course, all these groups necessitate complementary functional groups for cross-linking and there are several strategies to bring these complementary functional groups into the dispersion system:                a) They can simply be incorporated into the same dispersion, i.e. complementary groups coexist within one latex particle. Evidently, a limited pot-life will be the consequence.        b) The complementary groups can be introduced into the water-phase where they would stay un-reacted until they get close to their counter-parts during film-formation.        
                c) Another route would be to synthesize two different latexes bearing the complementary functional groups and blending these latexes before application. The reactive groups would then be separated in the first place and could encounter upon film-formation, after inter-diffusion of the polymer chains.        

Recently, carbodiimide (CDI) chemistry has attracted interest as a cross-linking system for latex systems. The cross-linking reaction between carbodiimide and carboxylic acid occurs at ambient conditions forming predominantly N-acyl urea products according to the reaction shown below.

The reaction rate depends both on the structure of the CDI and on the acid. Aliphatic CDIs are reported to be highly reactive; whereas, their aromatic counterparts are less reactive. The reaction rate can be even further decreased by introducing steric hindrance near the CDI group. Finally, it is known that strong acids react at a slower rate with CDIs than weak acids, such as carboxylic acids, do. The resulting cross-linking rate will therefore depend on the interplay of all these parameters. Extra complications come into play by the process of film formation where the competition between polymer inter-diffusion and rate of cross-linking determine the final degree and topology of the polymer cross-links.
According to route b), it is desired that the maximum reaction rate of the cross-linking system to be slower than the rate of film formation in order to get complete property development in terms of hardness, scrub resistance, and solvent resistance.
Among the candidates of latexes with reactive groups, carboxylated latexes continue to be among the most desired for crosslinking films. This choice is due to several reasons: 1) carboxyl groups already cofunction in the surface stabilization of latex particles, and, therefore, do not have to be specially incorporated into the latex, 2) the reactivity of the carboxyl groups is very energetically favored at ambient temperatures, and 3) carboxyl groups can take part in a broad assortment of different crosslinking reactions with a wide variety of a coreactive groups (e.g. ionic crosslinkers). Out of this wide variety of coreactive groups, carbodiimides react with carboxylated functionalized latex coatings to provide the N-acyl urea crosslinks described above.
It would be desirable to obtain a composition from a carboxylated latex crosslinked with a carbodiimide that has desired properties in a film formed from the composition.